1. Field of the Invention
The present invention relates to a heat exchanger such as an oil cooler, having an outer cylindrical pipe (first tube) and an inner cylindrical pipe (second tube), for cooling engine oil, hydraulic oil (ATF) for an automatic transmission, or the like (hereinafter simply referred to as "oil").
2. Description of Related Art
An oil cooler having an outer cylindrical pipe and an inner cylindrical pipe is structured, as shown in JP-U-58-52462, such that a passage through which oil flows is formed between the outer cylindrical pipe and the inner cylindrical pipe and an inner fin is disposed in the passage. Generally, both pipes employ seamless pipes having no seam (connecting surface), which are produced by drawing or extruding, in view of mechanical strength, manufacturing cost, and the like.
To improve heat-exchange (cooling) efficiency, it is necessary to certainly contact the inner fin closely to both pipes. Therefore, generally, after the inner fin is inserted into the passage (gap), the inner cylindrical pipe is enlarged (this work is hereinafter referred to as "enlarging pipe work") by applying pressure from inside the inner cylindrical pipe before being brazed, so that the inner fin certainly contacts both pipes closely.
However, the enlarging pipe work needs a specific jig thereof. In addition, it is difficult to enlarge the inner cylindrical pipe uniformly; and therefore, a reduction of manufacturing cost of the oil cooler is disturbed.
Further, it is actually difficult to inspect whether or not the inner fin certainly contacts both pipes closely after the enlarging pipe work is finished. Therefore, when the enlarging pipe work is improper, the inner fin and both pipes may be brazed to each other while forming a gap therebetween, and a deterioration of the heat-exchange efficiency and the durability may be caused.